The present application relates to memory structures which exploit the hysteretic and single-electron-gated conduction properties of isolated-granular films.
In the last twenty years integrated circuits have become continuously smaller and faster, and they consume less power per device. As further development of the well established CMOS technology starts to reach its limits, more and more new technologies are being suggested which promise to continue the trend to smaller, faster, lower-power, and cheaper. Among these new technologies, single-electronics is particularly interesting. An introduction to single-electron devices can be found in the article by Konstantin Likharev and Tord Claeson, xe2x80x98Single Electronicsxe2x80x99, Scientific American, Jun. 1992, p. 50-55, which is hereby incorporated by reference.
Single-electron devices and circuits include islands (electric conducting regions such as quantum dots, metal grains and metal stripes) which are separated by tunnel barriers (potential barriers such as oxide). Additionally capacitors, resistors and voltage sources are incorporated. Islands (quantum dots) can only be charged or discharged by tunneling through potential barriers or thermionic emission over potential barriers. Such a charging can be suppressed by the so called Coulomb blockade, particularly if tunnel junction and island are very small. The Coulomb blockade together with tunneling allows the controlled transfer of single electrons between quantum dots (islands). This is also the reason for the name xe2x80x98single-electronicsxe2x80x99. The Coulomb blockade is the central physical effect which makes single-electronics possible. A number of devices (e.g. transistors, memories, logic gates, pumps, electrometers, among others) and manufacturing methods relating to this technology have been published or patented: see for example PCT and EPC applications WO94/15340, EP0642173A1, EP0727820A1, EP0675546A2, EP0718894A2, EP0750353A2, WO96/16448, and EP0649174A1, all of which are hereby incorporated by reference.
Crucial for an industrial success of single-electronics is operation at room temperature, manufacturability with established, economical, and reproducible methods, immunity to unavoidable manufacturing tolerances, and immunity to upset by random background charges. All devices so far published violate one or more of the above stated requirements (room temperature operation, simple mass fabrication, and immunity to random background charges).
To achieve a room temperature operation with such conventional devices, one would need to produce structures smaller than 10 nm. Fabrication tools for mass-producing such small structures are not available today. Unavoidable impurities introduce random background charges which can possibly suppress the Coulomb blockade entirely and thus break the desired operation of single-electron devices and circuits.
Immunity to random background charges is a particularly important requirement, since any semiconductor structure will have a certain density of free carriers at room temperature, any real semiconductor interface will have a certain density of traps, and any real environment will have a certain flux of ionizing radiation. The sensitivity which allows the device to respond to a single electron will also allow it to be affected, undesirably, by random charges.
This application describes a single-electron memory device which is immune to random background charges, works at room temperature, and is mass manufacturable with established and tested methods.
The disclosed devices use at least two isolated-granular films which are capacitively coupled. It solves all three above mentioned requirements for a digital memory element in a simple manner. A grain size below 10 nm is achieved by employing naturally formed granular films, particularly isolated-granular metal or semiconductor films, which are used in thin film structures. The size and shape of these films, rather than the individual grain, can then further be defined by well established lithography methods (optical lithography, e-beam) or any future method (SCALPEL, x-ray, ion-beam, near field, etc.). The smallest entity is thus not anymore the individual grain or tunnel barrier, but an isolated-granular film consisting of several nanoscopic grains and tunnel barriers. Since isolated-granular films can be produced with grain sizes down to 1 nm or less, the room temperature operation is ensured. And since there is no need to define individual quantum dots lithographically (they form naturally) there is no need for not yet established and available nanolithographic manufacturing tools. Thus patterning methods with minimum structures around 100 nm or even coarser are good enough for a successful production of the inventions described in this application. Additionally, employing isolated-granular films rather than individual quantum dots introduces a beneficial averaging effect which reduces the sensitivity to random background charges of this device. Background charges which disturb an individual island or a small part of the isolated-granular film do not necessarily have to disrupt the whole isolated-granular film. Hence the inventions described in this application can deal much better with impurities and errors introduced during manufacturing.
Further, this memory device can be read by Coulomb oscillations, as will be described later in more detail, which is insensitive to random background charges. This further reduces susceptibility to random background charges. The fact that the inventions described in this application employs isolated-granular films introduces therefore a strong resistance against random background charges which are caused by impurities and manufacturing errors.
A second reading mechanism exists where the IV-characteristic of one isolated-granular film changes depending on the charge stored in the device.
In contrast to the structures according to patent EP 0 642 173 A1 and EP 0 727 820 A1, where in one granular film storage and readout happens, the inventions described in this application has separated storage and readout structurally. This allows a separate optimization of the part which stores charges (information) and the one which reads it out. This means that the granular film responsible for storage and the granular film responsible for readout could be manufactured from two different materials and could be processed with two different process steps.
The properties of the isolated-granular film responsible for storage could be adjusted so that a nonvolatile memory device is achieved. This requires in particular high resistive isolated-granular films.
As already mentioned, the thin isolated-granular films in the inventions described in this application could be produced, for example, by evaporation or epitaxial growth methods followed by lithographic steps. Beside evaporation onto a substrate, one could also incorporate isolated-granular films into semiconducting structures, which is a well known technique today.